There are known fixing apparatus to be installed in a copying machine or a printer, including an endless belt, a ceramics heater that is in contact with an inner surface of the endless belt, and a pressure roller for forming a fixing nip portion together with the ceramic heater via the endless belt. When small size sheets are successively printed in an image forming apparatus having such a fixing apparatus installed therein, there occurs a phenomenon (temperature rise in a non-sheet feeding portion) in which a temperature gradually increases in an area having no sheet to pass therethrough, in a longitudinal direction of the fixing nip portion. If the temperature in the non-sheet feeding portion is increased to be too high, each part in the apparatus may be damaged. Further, when a large size sheet is printed under a state in which a temperature rise is caused in the non-sheet feeding portion, a hot offset of toner may occur in an area corresponding to the non-sheet feeding portion for a small size sheet.
As a method of suppressing the temperature rise in the non-sheet feeding portion, there is conceived a method in which heat generation resistors on the ceramic substrate are each made of a material having a positive resistivity-temperature characteristic and two conductive members are disposed on both ends of the substrate in the transverse direction of the substrate so that current flows through in the transverse direction (recording sheet conveyance direction) of the heater with respect to the heat generation resistors. The method is based on an idea that, when the temperature in the non-sheet feeding portion rises, the resistivity of each of the heat generation resistors in the non-sheet feeding portion is increased so as to suppress current flowing through the heat generation resistors in the non-sheet feeding portion, to thereby suppress heat generation in the non-sheet feeding portion. The positive resistivity-temperature characteristic refers to a characteristic that the resistivity increases along the increase in temperature, which is hereinafter referred to as positive temperature coefficient (PTC).
However, a material having PTC is significantly low in volume resistance, and hence it is extraordinary difficult to set the total resistance of the heat generation resistors in one heater to fall within a range for use at commercial power. In view of this, PTL 1 discloses the following configuration. That is, the heat generation resistors of PTC to be formed on the ceramic substrate are divided into multiple heat generation blocks in a longitudinal direction of the heater, and, in each heat generation block, two conductive members are disposed on both ends of the substrate in the transverse direction so as to allow current to flow in the transverse direction (recording sheet conveyance direction) of the heater. Further, the multiple heat generation blocks are electrically connected to one another in series. PTL 1 further discloses that the multiple heat generation resistors are electrically connected in parallel to one another between the two conductive members, to thereby form each of the heat generation blocks.